In a front longitudinal configuration of a drive unit consisting of a drive engine and a transmission in a motor vehicle, the output shaft of the transmission is preferably arranged in a common geometrical axis with the input shaft or input shafts of the transmission. In the case of a dual-clutch transmission with a centrally arranged first input shaft and a second input shaft in the form of a hollow shaft fitted coaxially over the first input shaft, the coaxial arrangement of the output shaft required for this can be achieved if, by way of a respective input constant in each case, the input shafts are or can be brought into driving connection with an associated countershaft. To engage the gears, the countershafts can each be brought into driving connection with the output shaft by means of a number of spur gear stages, each of which can respectively be engaged by means of a gearshifting element. Furthermore, with this arrangement a direct gear can usually be engaged by connecting the first input shaft to the output shaft in a rotationally fixed manner.
In a generally known first basic design of a dual-clutch transmission of this type, the countershafts are arranged axis-parallel with and a distance away from one another and from the output shaft. In an embodiment of a dual-clutch transmission known from U.S. Pat. No. 4,463,621 B1, the shiftable spur gear stages are arranged in various gearwheel planes. The loose wheels of the spur gear stages are in each case mounted to rotate on the output shaft and can be connected in a rotationally fixed manner to the output shaft by means of the associated gearshifting elements. By virtue of a coupling shifting element arranged between the central, first input shaft and the output shaft, a direct gear can be engaged. For four forward gears and one reverse gear, this known dual-clutch transmission requires a total of six gearwheel planes.
In contrast to this, in a dual-clutch transmission known from DE 103 35 262 A1 there are five spur gear stages and one reversing spur gear stage, each using a common fixed wheel or loose wheel arranged on the output shaft in pairs in common gearwheel planes. The two outer gearwheels of the reversing spur gear stage are loose wheels, whereby two out of three possible reverse gears can be engaged as winding gears. For seven forward gears and three reversing gears this known dual-clutch transmission has a total of seven gearwheel planes.
In a generally known second basic embodiment of such a dual-clutch transmission the second countershaft associated with the second input shaft is arranged axis-parallel with and a distance away from the output shaft, and the first countershaft associated with the first input shaft is in the form of a hollow shaft arranged coaxially over the second countershaft.
In the dual-clutch transmission of this type known from DE 199 39 819 C1 the shiftable spur gear stages are arranged in different gearwheel planes. The loose wheels of the spur gear stages are in each case mounted to rotate on the output shaft and can be connected rotationally fixed thereto by the associated gearshift elements. By means of a coupling shifting element arranged between the centrally positioned first input shaft and the output shaft, a direct gear can be engaged. For six forward gears and one reverse gear this known dual-clutch transmission needs a total of eight gearwheel planes.
In contrast, in a further embodiment of a dual-clutch transmission of this type known from DE 103 32 210 A1 the drive output wheel of the first input constant associated with the first input shaft is in the form of a loose wheel, which is mounted to rotate on the first countershaft and can be connected rotationally fixed thereto by means of a first coupling shifting element. In addition the first countershaft can be connected rotationally fixed to the second countershaft by means of a second coupling shifting element. By means of the two coupling shifting elements, the spur gear stages arranged between the first countershaft and the output shaft can be alternately brought into driving connection with the first or second input constant and thus used, in each case, for the engagement of two gears. For six forward gears and one reverse gear, this known dual-clutch transmission has a total of six gearwheel planes.